I agree with this - guesswork and invisible options can be confusing.
That's why I suggest what I think is the simplest solution: Just let
this be overridable on the command line.

Isn't the user-net IP irrelevant to the outside? AFAIK, it just causes
Qemu to act as a normal TCP/IP client to the OS it's running on, and the
guest OS simply can't accept incoming connections (nobody actually knows
that the program issuing the connections is actually hosting an OS
inside).